A non-linear optical effect is strikingly displayed when exposing a material to intensive light such as laser beams. Such effect can be applied to a second-order higher harmonic wave generation, i.e., SHG, a third-order higher harmonic wave, i.e., THG, an intensity modulation, a switching operation, an optical memory, and so forth. The application of the non-linear optical materials having non-linear optical effect to the optical devices of the next generation have focussed the spotlight of attention.
In recent years, many reasearches and developments of such non-linear optical materials have energetically been progressed. Especially about an SHG effect, it has been pointed out that organic type materials have a possibility to display extraordinarily higher efficiency in non-linear optical constant, a response speed, durability, and so forth, as compared to conventional inorganic type materials such as lithium niobate LiNbO.sub.3, potassium dihydrogenphosphate KDP, and so forth which have been well known. Such an indication as mentioned above is found out in, for example, `Organic Non-linear Optical Materials`, compiled under the supervision of Masao Kato and HachirO Nakanishi, published by C. M. C. Company, Japan, 1985.
In the meantime, the crystal growth of non-linear optical materials, the devices using the non-linear optical materials, and so forth have also been studied extensively. In particular, there have been strong demands for obtaining an optical wave guide device capable of serving as a frequency conversion device utilizing SHG or THG, displaying a high conversion efficiency, and emitting uniform-phase rays of light.
The compounds having an SHG effect include, for example, 2-methyl-4-nitroaniline (MNA), 2-acetoamido-4-nitro-N,N-dimethylaniline (DAN), 2-acetoamido-4-nitro-1-pyrrolidinobenzene (PAN), 2-(.alpha.-methylbenzyl)amino-5-nitropyridine (MBA-NP), and so forth, each of which have been well known.
The above-given compounds are excellent materials among those having the SHG effect, because of the great non-linear optical effect. However, the organic non-linear optical materials typified by the above-given compounds can hardly obtain single crystals having a satisfactory size for serving as the core of the optical wave guide. It has therefore been very difficult to make them be the optical wave guide so as to serve as frequency conversion devices.
For example, a conventional fiber type optical wave guide device has been prepared in such a method that, after a highly refractive non-linear optical material is fused and injected into a hollow fiber made of a low refractive material, the crystallization thereof is carried out in Bridgman-Stockbarger method or the like. For further details, refer to D. S. Chemla., J. Zyss: Nonlinear Optical Properties of Organic Molecules and Crystals, Vol. 1; Academic Press, Inc., (1987).
However, it is generally said that the excellent core of an optical wave guide is difficult to prepare by making use of the conventionally known organic non-linear optical materials capable of displaying an excellent non-linear optical effect, because single crystals can hardly be formed in such a method as mentioned above. For example, PAN and MBA-NP may be unable to form any single crystal, because these materials are decomposed when they are fused.
Optical recording media are being studied extensively to try to satisfy the demans for making mass-storage and high-density information recording media. The recording density of these optical recording media depend on the wavelength of a light-source used, i.e., the recording density limit is increased at the inverse square of the wavelength of the light-source when the wavelength is shortened. Therefore, frequency conversion devices are greatly expected to obtain a light-source having a wavelength more shorter than ever. For example, it is expected to obtain blue rays of light from near-infrared rays light, light that is in the region of emitting semiconductor laser beam. However, most of the conventionally excellent organic non-linear optical materials such as MNA and DAN are yellow-colored and low in blue light transmittance. Such conventional materials have been disadvantageous to serve as a frequency conversion device capable of emitting blue rays of light, because they have a low transmittance of blue rays of light.
The above-mentioned facts have led a strong demand to the realization of organic non-linear optical materials which are ready to form single crystals and high in optical transmittance in the areas of visible rays of light including, especially, blue rays of light.
The original source of the non-linearity of an organic compound is an intramolecular .pi. electron, and polarizability .beta. of a second-order non-linear molecule becomes particularly greater when the compound has both of an electron-donative group and an electron-attractive group.
However, as is typified by p-nitroaniline, even if a non-linear polarization is great in molecular level, there may be many crystallized states where no SHG is shown at all or only a few SHG are shown. It is, therefore, impossible to judge whether a compound can show an excellent non-linear optical effect or not, by the polarizability .beta. of the second-order non-linear molecule used.
The present inventors studied the non-linear optical effect of compounds, the crystallization thereof, and the transmittance of visible rays of light. They have resultingly discovered the compounds each capable of displaying an excellent non-linear optical effect, eliminating the absorption of visible rays of light, and forming the crystals having a size good enough to readily make an optical wave guide.